1. Field of the Invention
The invention relates generally to downhole instruments used to transmit an indication of the occurrence of event(s). More particularly, the invention relates to fluid pressure modulation telemetry systems used with such instruments to transmit the indications.
2. Description of the Related Art
Drilling and completion systems known in the art include so called measurement-while-drilling (MWD) systems. MWD systems include one or more sensors disposed in an instrument lowered into the wellbore, typically during the drilling, completion, or treatment thereof, which detect a physical parameter related to a condition in the wellbore or to a property of the formations surrounding the wellbore. MWD systems also include electronic circuitry which converts the measurements made by the one or more sensors into a representative signal which is applied to some form of fluid pressure modulation telemetry. Pressure modulation telemetry uses a device to alter the flow of drilling or treatment fluid through the instrument in a predetermined manner to communicate the representative signal to the earth""s surface. The signal is detected typically by one or more pressure sensors disposed at the earth""s surface in the fluid circulation system. A detection, interpretation and recording system coupled to the pressure sensor decodes the representative signal to extract the measurement made by the one or more sensors. Typical MWD systems are described, for example, in U.S. Pat. Nos. 3,958,217; 3,964,556; 3,736,558; 4,078,620; and 5,073,877.
A problem common to all prior art MWD pressure modulation telemetry systems is pressure noise in the fluid circulation system. Such noise can be caused by, among other things, pulsations in the output of the fluid circulation pump, and vibrations and shocks caused by the movement of the drilling equipment (and consequently the instrument itself). Pressure noise can make detection of the MWD telemetry signal difficult, particularly at high data rates. It is common in MWD telemetry to represent the value of the representative signal as a binary coded decimal xe2x80x9cwordxe2x80x9d including a number of digital bits related to the measurement range for the particular one of the sensors represented in the telemetry signal. As is known in the art, various modulation techniques are applied to the fluid pressure to represent digital xe2x80x9conesxe2x80x9d and xe2x80x9czeroesxe2x80x9d in the telemetry. Typical modulation techniques include momentary pressure increases (positive pulse telemetry), momentary pressure decreases (negative pulse telemetry) and phase shift keying of a standing wave (mud siren).
Detection of the proper sequence of binary coded information to recover the representative signal is difficult in noisy conditions, and may require expensive and difficult to operate equipment at the earth""s surface. Further, the typical telemetry generator used in MWD systems is expensive to make and to operate. Finally, detection of certain types of downhole conditions can be represented by more simple telemetry signals than are provided in the typical MWD telemetry system.
One solution to the limitations of conventional MWD telemetry for use in transmitting simple indications of a downhole condition is described, for example, in U.S. Pat. No. 5,626,192 issued to Connell et al. The device described in this patent is a casing collar locator which is adapted to be operated at the end of a string of coiled tubing. A casing collar detector in the instrument conducts electrical signals to a controller in the instrument, which upon receipt of a collar detection signal, operates a valve consisting of a set of lateral ports. The ports, when opened, conduct some of the fluid flowing through the instrument to the annular space between the outside of the coiled tubing and the wellbore wall. While the instrument in the Connell et al ""192 patent has proven effective, there are circumstances where diverting fluid flow from the interior of the tubing/instrument to the annular space outside them is undesirable. Such circumstances include, but are not limited to, setting a plug or pumping acid or scale removal chemicals through the coiled tubing and the instrument.
What is needed is a fluid pressure telemetry system which provides robust, easy to detect signals at the earth""s surface, and maintains fluid flow within the instrument.
One aspect of the invention is a system for communication from an instrument disposed in a wellbore. The system includes a flow diverter selectively operable to conduct fluid flow through a first path along the interior of a housing and a second path along the interior of the housing. The system includes an initiator operatively coupled to the flow diverter to cause selective operation thereof in response to a first event.
The first event can comprise any of a number of occurrences, including but not limited to, the detection of certain downhole components, the sensing of certain wellbore conditions, the sensing of certain tool string or tool component conditions, the sensing of certain formation characteristics, the expiration of a period of time, the execution of a software program or subroutine, or the reception or transmission of a signal from or to components at the surface or in the wellbore. Depending on the nature of the first event, the initiator may also include at lease one detector, software program, analyzer, timer, or sensor (to name a few) in order to sense the occurrence of the first event. Generally, when the initiator senses the first event, the flow diverter diverts at least some of the fluid flow to the second flow path, which creates a pressure change that can be sensed and that serves as an indication of the occurrence of the event.
In one embodiment, the flow diverter is a piston operated by an actuator. One embodiment of the actuator is a ball screw operated by an electric motor. One embodiment of the initiator is operatively attached to a casing collar locator wherein the first event comprises the detection of a casing collar by the locator. Upon detection of a casing collar in the wellbore, the piston is moved from a first position to a second position, to divert flow from the first path to the second path, for a selected amount time, to indicate detection of the casing collar.
A method for communicating from an instrument disposed in a wellbore according to another aspect of the invention includes conducting fluid flow through a first path having a first flow restriction. The first flow path is located along the interior of the instrument. Upon the occurrence of a first event, the fluid flow is diverted along a second path having a second flow restriction in response to the first event. The second path is located along the interior of the instrument.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.